1. Field of the Invention
The present invention relates to a method of controlling the temperature of a recording head of an inkjet recording apparatus.
2. Description of the Related Art
For example, inkjet recording apparatuses which eject ink from a plurality of ink ejection holes are known as recording apparatuses which perform recording using recording heads having a plurality of recording elements. In an inkjet recording apparatus, the temperature of ink before ejection is an important parameter for stable ejection of ink in a predetermined amount from each ejection hole.
In general, physical properties of ink such as viscosity and surface tension are apt to change depending on the temperature which also affects the state of ejection. In particular, the viscosity of ink increases in a low-temperature environment, which can result in unstable ejection and a reduction in recording quality. In order to avoid this, most inkjet recording apparatus employ a configuration in which a heater is provided inside or outside of a recording head to heat ink to a predetermined temperature prior to recording.
When ejection does not take place for a predetermined time or more, the viscosity of ink increases due to evaporation of moisture in the vicinity of ejection holes, and the discharge condition may be consequently degraded especially for several initial shots of ejection. Hereinafter, such a discharge condition of several initial shots is referred to as “initial discharge condition”. A solution to this problem is to perform ejection a predetermined number of times until normal ejection is enabled in a place other than a recording position during recording or before the next recording is started. Such an ejecting operation is generally referred to as “preliminary ejection”. However, a problem still arises in that recording speed is reduced by preliminary ejection because it interrupts recording when performed during recording.
In order to minimize preparatory ejecting operations, it is advantageous to adjust the temperature of a recording head in advance using a heater as described above. The reason is that an increase in ink viscosity can be prevented by adjusting the ink in the recording head to a temperature within a predetermined range even if there is some evaporation of moisture.
Several methods have already been proposed and put in use to eject ink from inkjet recording apparatus. Above all, a method is frequently used in recent years in which: an electrothermal transducer (ejection heater) is provided in each of a plurality of ink channels leading to respective ejection holes; an electrical pulse is applied to them to cause film boiling; and ink is ejected from the ejection holes by bubbling energy thus generated. In an inkjet recording apparatus having such a configuration, heaters for keeping the temperature of the recording heads (hereinafter referred to as “sub-heaters”) are frequently provided separately from the ejection heaters used for ejection. The two types of heaters are provided on the same substrate that forms a part of the recording heads. In order to control the temperature of ink prior to recording, not only the sub-heaters but also the ejection heaters may be used in combination. In this case, the ink is warmed by the ejection heaters directly and by the sub-heaters indirectly before it is ejected.
When the ejection heaters are used as a heating source for the ink, such a short pulse that no bubbling is caused thereby is applied to the ejection heaters. The temperature of the recording heads is continually detected, and the application is continued until a predetermined temperature is reached and is stopped when the predetermined temperature is reached. The temperature of the recording heads is kept within the predetermined range through repetition of the process.
When the sub-heaters are used as a heating source for the ink, in general, the sub-heaters are continuously energized until the predetermined temperature is reached. Referring to the detection of the recording head temperature, the temperature of the ink may be detected either directly or indirectly. In either case, the energization of the sub-heaters is continued until the temperature of the recording heads thus detected reaches the predetermined temperature and stopped when it becomes equal to or higher than the predetermined temperature. The temperature of the recording heads is kept within the predetermined range through repetition of the process.
However, the temperature adjusting method utilizing ejection heaters and sub-heaters as described above still has the following problems to be solved.
For example, according to the temperature adjusting method utilizing ejection heaters, the temperature of ejection heaters can instantaneously increase to generate bubbles even when they are driven with such a short pulse that no ejection is caused thereby. Such bubbles accumulate inside the recording head and have a bad influence on subsequent ejections.
A short pulse for heating cannot be applied to the ejection heaters while a pulse for ejection is applied to them. While control for temperature adjustment may be conducted in short intervals between ejections there is concern about a reduction in the recording speed and complicatedness of the control in this case. An alternative is to perform temperature adjustment until immediately before recording and not to conduct control for temperature adjustment during recording. However, when there is a long non-recording period in a recording operation, the temperature of the recording heads can decrease below the value to be maintained.
When temperature adjustment is performed using sub-heaters, the power of the sub-heaters used can be a problem. When heaters of high power are used, control may not be conducted with stability because of great temperature ripples, although a target temperature can be reached quickly.
When heaters of low power are used, a target temperature can be maintained with reduced temperature ripples by turning the energization of the same on and off repeatedly. However, it takes a long time to reach the target temperature when a great temperature increase is needed, which can affect recording time.
A possible approach is to perform temperature adjustment before a recording start instruction is received and to perform ejection as soon as the recording start instruction is received in order to prevent any adverse effect on the time of recording since the point in time when the recording start instruction is received. However, there will be no change in the time spent before the temperature of the recording heads reaches the target temperature in practice. When there is a great difference between the target temperature and the ambient temperature, the viscosity of the ink in the recording heads increases as a result of an increase in the density of a dye in the ink because a great amount of moisture evaporates from the recording heads when they stand by, which can result in degradation of the initial discharge condition. Further, since the recording heads are kept at a relatively high temperature, the generation and growth of bubbles in the ink are promoted, and the ejection performance of the recording heads is therefore more vulnerable to adverse effects.
In order to avoid those problems, a method has already been proposed in which sub-heaters and ejection heaters are used in combination to perform temperature control. For example, the temperature is increased in a relatively short time using ejection heaters until it reaches a predetermined value that is lower than an actual target temperature and when the predetermined temperature is reached, the heating means is switched to sub-heaters to perform heating slowly up to the target temperature. In this case, however, a complicated circuit for simultaneously controlling the ejection heaters and the sub-heaters in the same recording head is required in occasions in which the recording head must be heated during recording such as when image data are sparsely distributed.